Profile extrusion processes have been developed over many years. For example, various types of pipe have been produced by extruding rigid PVC (polyvinylchloride), HDPE (high-density polyethylene) and ABS (acrylonitrile-butadiene-styrene). Complex profiles, such as window frame, siding, fencing and decking components have also been made with these processes, using, for example, rigid PVC. The manufacture of these products is conducted using polymers with no or relatively low filler content (less than 40% by weight).
The conventional profile extrusion process requires conditions where the temperature at the extrusion die is well above the melting/softening point of the polymer. The profile exits the die in molten form, and is received in a vacuum sizing tank/calibrator, which prevents the profile from collapsing. The vacuum sizing tank/calibrator comprises a solid structure with a bore therethrough matching the size and shape of the extruded profile. The dimension and shape of the profile is maintained by applying vacuum to the outer part of the profile while it is cooled.
In the conventional profile extrusion process, the sizing vacuum tank/calibrator is not attached to the die. Therefore, it is necessary that the profile exiting the die does not swell nor melt fracture because the vacuum sizing tank/calibrator cannot accommodate extrudate which has undergone die swell and melt fracture. Die swell and melt fracture are typical phenomena in profile extrusion when the temperature of the polymer in the die is too low and/or the viscosity of the molten polymer/composite at the die is too high. This phenomenon is usually accompanied by high-pressure drop across the die. To prevent these problems profile extruders typically employ a puller downstream of the die. The puller draws down the gauge of the profile, counteracting the effects of die swell. The puller also lowers the die head pressure.
Known processes for the manufacture of composites with high filler content mainly involve compression molding, where a mixture of resin and filler is shaped in a mould by pressing the two parts of the mould together. Most of these processes use thermosetting resins, such as urea-formaldehyde resins and melamine-formaldehyde, as the binder/matrix, but some processes use thermoplastic resins.
With the thermosetting resins, the product can contain up to 95% by weight of filler, because the binder is in liquid form prior to a curing reaction. Mixing of such a liquid with the filler can be done in a conventional mixer. Particleboard (using wood particles) and MDF or medium density fibreboard (using wood pulp) are typical examples of such composites. The mixture is then compression molded into sheet/board (particleboard and MDF) or into various shapes, and heat is applied to cure the resin. Once the curing process is complete, the product is cooled and released from the mould. Products made from thermosetting resins are usually non-recyclable because thermosetting resin cannot be re-melted and re-shaped once it is cured.
Recently, interest has developed in completely recyclable products. Much effort has been put into replacing thermosetting resins with thermoplastic resins, especially those available in abundance in the post consumer recycling stream, such as fractional melt (high viscosity) HDPE from bottles and film (bags). With thermoplastic resins, melt-mixing (compounding) of the resin and the filler is required. Twin screw extruders and kneaders are most commonly used for this purpose, but they are limited to relatively low filler content or thermoplastic resins with relatively low viscosity. This excludes mixing of fractional melt HDPE with amounts of filler of 40% to 60% by weight.
At least three other processes have been proposed for melt-mixing or compounding thermoplastic resin with up to 80% by weight of filler. They are all based on a high speed, high shear thermokinetic mixing process. Once the compound is prepared by mixing it is shaped into the final product. Using compression moulding, this compound can be shaped into sheet/board and other shapes. Extrusion of such a compound has also been explored.
U.S. Pat. No. 5,516,472 to Laver discloses a process for combining an organic fibrous material with a thermoplastic material forming a wood imitation composite. The process comprises the steps of dry-blending the raw materials, melt blending them in an extruder, passing the homogenous mixture through a transition die to pre-shape the mixture and to expand the mixture, passing the mixture through a stranding die to form a plurality of strands, and finally passing the plurality of strands through a moulding die for a time sufficient to compress the strands together and bond the strands to each other. The preferred formulation of Laver's invention is approximately 65% wood flour, 26% high density polyethylene over 3% processing aid and over 5% thermoset.
U.S. Pat. No. 5,474,722 to Woodhams discloses a process to produce a high modulus article consisting of a composite of an oriented plastic material and an oriented particulate material. The orientation results from forcing the molten composite material through a converging passage to produce an extrudate, deforming the extrudate while maintaining the extrudate at or close to its melting temperature (1-10.degree. C. above the melting temperature) to produce an oriented deformed extrudate, and cooling the deformed extrudate to preserve the orientation.
PCT Publication WO 94/11174 to Suwanda et al. discloses a similar process to that of Woodhams, i.e. a process for continuous production of filled thermoplastic compound containing filler, having oriented components. The process comprises the steps of bringing the material to a molten stage, but at a temperature just above the softening temperature (0-10.degree. C. above the melting temperature), forcing the molten material through a converging die to impart longitudinal orientation to the polymer and the filler particulates, and cooling the compound to preserve the orientation.
The Laver process results in extrudate with poor structural characteristics due to the large amount of processing aid. Also the thermosetting components would degrade upon recycling of the material. This would cause the physical properties to degrade as well. Both processes of Woodhams and Suwanda claim to be able to extrude thermoplastic compound with up to 80% filler by weight into profiles. Both also concentrate on designing the converging flow through the die to control the elongational strain necessary to create the orientation of polymer molecules as well as the filler particulates.